RFID tags and readers that communicate using magnetic coupling or magnetically coupled transponders (e.g., according near field communication [NFC], ISO 14443 Proximity Smart Card, or ISO 15693 Vicinity Smart Card standards operating at 13.56 MHz, or variations thereof) perform well in the presence of most liquids. However, signal transmissions to and/or from such RFID tags generally degrade in the presence of metal.
Generally, in tag and reader applications, the tag is sometimes attached to an article that is on or in close proximity to a metallic surface. Although ferrite shielding may be effective in counteracting the effect of nearby metal objects on tags, ferrite shielding is relatively expensive, especially for relatively large antennas. Presently, there is intense pressure in the market to develop high volume, functionally reliable RF and NFC devices such as tags and readers at a relatively low cost. Mobile phones in ‘tag emulation’ mode work as tags. The NFC antenna inside the mobile phone may be positioned relatively close to metallic objects because of space constraints.
Alternatively, a gap (e.g., a spacer of non-conducting and/or non-magnetic material) may be positioned between the metal surface and the tag. However, spacers are not often desirable, available or permitted, due to space constraints.
Similar problems occur with reader antennas, such as antennas mounted on metallic shelves (e.g., “smart shelves”) used for automated inventory tracking. Referring to FIG. 1, an arrangement 10 for conventional automated inventory tracking with smart shelf technology is illustrated. Typically, high frequency (HF) systems provide better locational precision than ultra-high frequency (UHF) in “smart shelf” systems. However, a HF reader antenna 20 positioned on a metal shelf 30 requires spacers (not shown), which waste space and are inconvenient. Mobile phones in reader mode work as readers. The NFC antenna inside the mobile phone may be positioned relatively close to metallic objects because of space constraints.
Generally, mobile devices such as smart phones, tablet computers, and pad computers can act as either a tag or reader, and antennas in such mobile devices are subject to the same effect of nearby metal on tag/reader antennas, but this effect is exacerbated by the fact the antenna is constrained in a relatively small space or area.
Various alternatives to expensive ferrite based solutions exist. However, they are generally not practical and/or desirable. For example, perforated metal sheets may be an integral part of reader antennas, but may be relatively expensive and challenging to design and implement. Alternatively, anti-parallel loops may demonstrate improved performance in wireless power transmission (WPT) compared to conventional coils in the presence of or proximity of metals. However, this approach may require an impractical number of coil turns.
Extraneous objects (e.g., metal) in WPT may be treated as an impedance mismatch issue, and improvements in transmission efficiency (e.g., power gain) may be demonstrated by proper matching. However, the mechanism to achieve such impedance matching using this approach is not well understood.
FIG. 2 shows surface currents (e.g. eddy currents) 50 on a metal sheet 60 with an antenna 70 thereon. Generally, a magnetic field generated by such eddy currents 50 opposes an excitation field (e.g., which may be generated by the antenna 70). The total flux linked by the coil in antenna 70 thus decreases in such an environment. As a result, inductance decreases, and the resonant frequency of the antenna 70 increases (i.e. mistuning). Consequently, flux linked by a secondary loop (not shown), such as that in a corresponding device (e.g., a tag), decreases, resulting in performance degradation, such as deterioration in power and signal transfer.
When an electromagnetic surface, such as an electrically conducting sheet having a relatively low impedance, is placed in the vicinity of a loop containing current (e.g., a reader antenna), eddy currents are generated (e.g., as shown in FIG. 2). As a result, a magnetic field is generated in opposition to the original field, thereby reducing the flux through the loop. Reduction of flux, and therefore inductance, results in an increase of the resonant frequency. Furthermore, reduction of the effective magnetic field reduces power transfer to a load that is connected to a second loop magnetically coupled with the first loop. Thus, a degradation of the performance is observed for magnetically coupled reader-tag systems in the presence of metal.
The phenomenon of surface (eddy) currents can be studied using surface waves, whereby a surface impedance (Zs) can be defined by the following equation:
                              Z          s                =                              1            +            j                    σδ                                    (        1        )            where σ is the conductivity, δ is the skin depth of the metal, and j2 equals −1. The surface impedance is analogous to sheet resistance, and therefore an estimate of the impedance around a closed loop may be made using the principle of determining the resistance of a loop by dividing it into a number of squares of known dimensions (e.g., one of which may be skin depth of the metal δ).
FIG. 3 shows surface (eddy) currents on the metal sheet and illustrates a principle by which impedance of the sheet 60 can be estimated using squares a1, a2, a3, etc., along the closed loop direction of a current loop of squares (not drawn to scale, for illustration only). The sheet 60 is divided into are array of squares, from which concentric loops are designated. The total impedance of a given loop is Zs·N1, where N1 is equal to the length of the loop/δ. Another concentric loop (e.g., outside the first concentric loop) is denoted by adjacent squares b1, b2, b3, etc.
FIG. 4 shows an equivalent circuit model 100 for a conventional near field communication device in the presence of metal. The reader equivalent circuit 110 includes resistances R0 and R1, capacitance C1, and an inductance L, excited by a voltage V which drives a current I1 through the loop. The equivalent circuit 120 for the metal sheet (e.g., sheet 60 in FIG. 3) includes coupled inductance L3 and resistance R3. A voltage induced in inductance L3 due to the magnetic field generated in L generates a current I3 in the circuit 120. Input voltage (V) can be expressed by the following equation:V=[R1+R0+j(ωL1−1/ωC1)]·I1−jωM13·I3  (2)in which 0=[R3+jR3]·I3−jωM13·I1, and the current (I3) is calculated by the following equation:
                              I          ⁢                                          ⁢          3                =                                                            1                +                j                            2                        ·                                          ω                ⁢                                                                  ⁢                M                ⁢                                                                  ⁢                13                                            R                ⁢                                                                  ⁢                3                                              ⁢          I          ⁢                                          ⁢          1                                    (        3        )            
FIG. 5 illustrates the mechanism of shielding using ferrite by changing the direction of magnetic flux density B towards the metal sheet 60 in the presence of a ferrite shield 150. However, ferrite shields generally add cost to the use of tags, and can introduce implementation issues in some cases. Therefore, a low cost solution is necessary to counteract and/or mitigate the effect of metal on or in proximity of magnetically coupled near field communication devices.
This “Discussion of the Background” section is provided for background information only. The statements in this “Discussion of the Background” are not an admission that the subject matter disclosed in this “Discussion of the Background” section constitutes prior art to the present disclosure, and no part of this “Discussion of the Background” section may be used as an admission that any part of this application, including this “Discussion of the Background” section, constitutes prior art to the present disclosure.